Field of the Invention
The invention relates to a steam turbogenerator set having a steam turbine unit and a driven machine unit connected thereto for producing electrical power. The invention also relates to a method for operation of the steam turbogenerator set.
Turbogenerator sets are generally used in order to feed an electrical network having a frequency which is 50 Hz (or 60 Hz). At high ratings (around 30 MVA or more), it is economical to operate the steam turbine at speeds of 3000 (or 3600) revolutions per minute when using two-pole generators. However, for lower ratings, higher speeds of more than 3000 to 16,000 revolutions per minute are more economical for the turbine, depending on the rating. In that case, a gearbox is required to reduce the speed between the steam turbine which rotates at high speed, and the generator which rotates to match the desired electrical power frequency.
In that case, particular problems occur with the lubrication and cooling of the bearings and gearbox.
In the gearbox, not only do bearings for gearbox shafts require special lubrication but, in particular, heavily loaded gearbox teeth of mutually engaging tooth edges must also be carefully lubricated and cooled. The high rotation speeds and loads in each case demand a specific coolant and lubricant, for which purpose, until now, only oils have been available, in practice.
Conventionally, a steam turbogenerator set has an oil circuit which essentially carries out three tasks:
Firstly, the oil is used as a lubricant and coolant for the bearings of the steam turbine and generator. Secondly, the control valves of the steam turbine are operated by actuating cylinders using oil for hydraulic purposes. Thirdly, the oil is used to cool and lubricate the gearbox. Heat losses which occur in each case are emitted to the oil circuit and are carried away to an oil/water heat exchanger. Overall, relatively large amounts of oil are required to carry out those three tasks. In such a case, the ratio of lubricating oil: control oil: gearbox oil is about 1:6:2.
Those amounts of oil can lead to a number of problems. In the event of leakages in the oil circuit, there is a risk of the oil which emerges contaminating the surrounding area. That necessitates precautionary measures such as oil trays and walls surrounding oil containers. Furthermore, emerging oil represents a serious fire hazard. If the oil comes into contact with parts of the turbine at temperatures up to 500.degree. C., there is a high probability of ignition. Alternative liquids which can be used are difficult to ignite but are generally toxic. Complex and expensive measures are required for steam turbine shaft bearings, particularly for steam turbogenerator sets with an axial stream outlet flow, to ensure that no oil can enter the outlet stream from the turbine. That would result in the steam circuit being contaminated by an extraneous medium, which could lead to a wide range of defects.
The amount of oil in the oil circuit can admittedly be reduced considerably if actuating cylinders operated hydraulically by oil are dispensed with and a change is made to a different medium (which then requires its own circuit) or different drive principles for the control valves (for example linear drives which, in some circumstances, likewise require cooling). However, that does not avoid impurities occurring in the steam outlet as a result of bearing oil emerging from the turbine, or oil emerging into the surrounding area. A high level of technological complexity is required for that purpose, as is implicitly evident from numerous patent applications (for example European Patent Application 0 306 634 A2, International Publication No. WO 94/01713 and German Published, Non-Prosecuted Patent Application DE 196 06 088 A1). That problem can be solved by shafts with magnetic bearings (for example in German Patent DE 42 27 280 C1, corresponding to U.S. patent application Ser. No. 08/390,107, filed Feb. 17, 1995, or German Patent DE 31 46 354 C2) or by other magnetic bearings with elements using permanent magnets and/or superconductors (German Published, Non-Prosecuted Patent Application DE 44 44 587 A1, corresponding to U.S. patent application Ser. No. 08/876,655, filed Jun. 16, 1997), although those likewise involve complexity. However, no promising substitute which operates without any coolant is yet known for the gearbox.